NGC 720: An elliptical galaxy located
about 80 million light years from Earth in the Constellation
Cetus.(Credits: X-ray: NASA/CXC/UCI/D.Buote et al., Optical: DSS
U.K.Schmidt Image/STScI)

Caption: Chandra's image reveals a
galaxy enveloped in a slightly flattened or ellipsoidal cloud of
7 million-degree Celsius gas. The shape and orientation of the
hot cloud are different from that of the optical image of the
galaxy. This means that a massive halo of dark matter is needed
to keep the hot gas from expanding away. Alternate theories of
gravity that eliminate the need for dark matter cannot explain
the observed shape and orientation of the cloud. The Chandra
data also fit predictions of a cold dark matter model. According
to this model, dark matter consists of slowly moving particles
which interact with each other and "normal" matter only through
gravity.